82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
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Application Note (AP-468)
4.4.2
Impedance Discontinuities
Impedance discontinuities cause unwanted signal reflections. Avoid vias (signal through holes) and
other transmission line irregularities. If vias must be used, a reasonable budget is two per
differential trace. Unused pads and stub traces should also be avoided.
4.4.3
Reducing Circuit Inductance
Traces should be routed over a continuous ground plane with no interruptions. If there are vacant
areas on a ground or power plane, the signal conductors should not cross the vacant area. This
increases inductance and associated radiated noise levels. Noisy logic grounds should be separated
from analog signal grounds to reduce coupling. Noisy logic grounds can sometimes affect sensitive
DC subsystems such as analog to digital conversion, operational amplifiers, etc. All ground vias
should be connected to every ground plane layer; similarly, every power via should be connected to
all equal potential power plane layers. This helps reduce circuit inductance. Another
recommendation is to physically locate grounds to minimize the loop area between a signal path
and its return path. Rise and fall times should be as slow as permissible because signals with fast
rise and fall times contain many high frequency harmonics, which can radiate significantly. The
most sensitive signal returns closest to the chassis ground should be connected together. This
results in a smaller loop area and reduce the likelihood of crosstalk. The effect of different
configurations on the amount of crosstalk can be studied using electronics modeling software.
4.4.4
Signal Isolation
To maintain best signal integrity, keep digital signals far away from the analog traces. If digital
signals on other PCB layers cannot be separated by a ground plane, they should be routed at right
angles with respect to the differential pairs. If there is another LAN controller on the PCB, take
care to keep the differential pairs from that circuit away.
